Donald Trump has the boldest approach to the problem. His comments on immigration and border security almost immediately became the leading headline of his campaign announcement speech earlier this year and the issue propelled him into the national conversation.

Trump says the 11 million people who are in the United States without documentation should be kicked out of the country. He would let some come back eventually, but all would have to leave. He also wants Mexico to help pay for a border wall to prevent more undocumented immigrants from crossing the border in the future.

Other candidates -- including Jeb Bush, Marco Rubio, Carly Fiorina and Rand Paul -- want to more strictly enforce existing immigration laws but also support a path to legal status for those who are already here.

"After we seal the borders, after we turn off the spigot that dispenses all the goodies," he said. "People who had a pristine record we should consider allowing them to become guest workers primarily in the agricultural sphere."

SIMI VALLEY, CA - SEPTEMBER 16: : Republican presidential candidates Mike Huckabee and Marco Rubio take part in the presidential debates at the Reagan Library on September 16, 2015 in Simi Valley, California. Fifteen Republican presidential candidates are participating in the second set of Republican presidential debates. (Photo by Justin Sullivan/Getty Images)

SIMI VALLEY, CA - SEPTEMBER 16: Republican presidential candidates Carly Fiorina and John Kasich take part in the presidential debates at the Reagan Library on September 16, 2015 in Simi Valley, California. Fifteen Republican presidential candidates are participating in the second set of Republican presidential debates. (Photo by Justin Sullivan/Getty Images)

SIMI VALLEY, CA-SEPTEMBER. 16: Republican presidential candidate Rand Paul take part in the presidential debates at the Reagan Library on September 16, 2015 in Simi Valley, California. Fifteen Republican presidential candidates are participating in the second set of Republican presidential debates. (Photo by Justin Sullivan/Getty Images)

Carly Fiorina, former chairman and chief executive officer of Hewlett-Packard Co. and 2016 Republican presidential candidate, speaks during the Republican presidential debate at the Ronald Reagan Presidential Library in Simi Valley, California, U.S., on Wednesday, Sept. 16, 2015. After a friendly round of introductions, the second Republican presidential debate quickly plunged into clashes that shifted from policy to the personal, with more politically experienced insiders trying to gain an advantage against the front-runner at center stage: Donald Trump. Photographer: Max Whittaker/Pool via Bloomberg

SIMI VALLEY, CA - SEPTEMBER 16: Republican presidential candidates Chris Christie (R) and Ohio Gov. John Kasich take part in the presidential debates at the Reagan Library on September 16, 2015 in Simi Valley, California. Fifteen Republican presidential candidates are participating in the second set of Republican presidential debates. (Photo by Justin Sullivan/Getty Images)

SIMI VALLEY, CA - SEPTEMBER 16: Republican presidential candidates Donald Trump (L) and Jeb Bush take part in the presidential debates at the Reagan Library on September 16, 2015 in Simi Valley, California. Fifteen Republican presidential candidates are participating in the second set of Republican presidential debates. (Photo by Justin Sullivan/Getty Images)

SIMI VALLEY, CA-SEPTEMBER. 16: Republican presidential candidate Donald Trump takes part in the presidential debates at the Reagan Library on September 16, 2015 in Simi Valley, California. Fifteen Republican presidential candidates are participating in the second set of Republican presidential debates. (Photo by Justin Sullivan/Getty Images)

2016 Republican presidential candidates Carly Fiorina, former chairman and chief executive officer of Hewlett-Packard Co., and Scott Walker, governor of Wisconsin, walk on stage during the Republican presidential debate at the Ronald Reagan Presidential Library in Simi Valley, California, U.S., on Wednesday, Sept. 16, 2015. The main debate of the top 11 GOP contenders in the polls follows the 'kids' table' debate of candidates who didn't make the cut. Photographer: Patrick T. Fallon/Bloomberg via Getty Images

SIMI VALLEY, CA - SEPTEMBER 16: Republican presidential candidate Donald Trump speaks with reporters after the presidential debate at the Reagan Library on September 16, 2015 in Simi Valley, California. Fifteen Republican presidential candidates are participating in the second of presidential debates(Photo by Sandy Huffaker/Getty Images)